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The ancestry of Homo floresiensis

Did ‘hobbit people’ of Flores evolve from Homo erectus or a more primitive hominin?

Homo floresiensis is an extinct Late Pleistocene hominin species known only from the Indonesian island of Flores. The type specimen LB 1 is a diminutive 30-year-old female who stood just 1.06 m (3 ft. 6 in.) tall. Nicknamed ‘Flo’, she had a cranial capacity initially estimated to be just 380 cc, comparable to that of an australopithecine. Her weight was estimated to be somewhere between 16 and 36 kg (35 and 79 lb.). Yet she was apparently human: she lacked the large back teeth of an australopithecine, the proportions of her facial skeleton were those of a human, and she appeared to be a humanlike fully-committed biped (Brown, et al., 2004; Morwood, et al., 2004).
The extremely small cranial capacity has been hotly debated since the species was first described in 2004. Some have claimed that Flo was a modern human suffering from microcephaly, a developmental disorder leading to a smaller brain (Jacob, et al., 2006; Martin, et al., 2006), but the majority reject this view and recognise Homo floresiensis as a new human species with a long, low cranial vault and other features characteristic of archaic humans (Argue, et al., 2006; Falk, et al., 2005; Falk, et al., 2007; Tocheri, et al., 2007; Lyras, et al., 2008).

Two principle theories have emerged as to the origin of these hominins. The first is that Homo floresiensis was a dwarf form of Homo erectus (approximate cranial capacity 1,000 cc) which underwent a dramatic reduction in size as a result of a phenomenon known as insular dwarfism. Animals living on an island where food is relatively scarce and predators are few or absent will ‘downsize’ over many generations, in order to reduce calorific requirements. What is actually happening is that evolution is favouring the smaller offspring in each generation. If predators do not pose a threat, any advantages in being large will be outweighed by poorer fuel-economy. What has to be questioned is whether insular dwarfism could lead to brain size reduction of the extent seen in Homo floresiensis.

The second theory is that Homo floresiensis is derived from a hominin species more primitive and smaller-brained than Homo erectus such as Homo habilis (approximate cranial capacity 600 cc) or even an australopithecine (approximate cranial capacity 400 cc). This second model implies that Homo erectus was not the first hominin species to leave Africa, contrary to the widely-accepted Out of Africa 1 hypothesis.

Two studies, one published in 2010 and the other earlier this year, have focussed on decreases in brain size. The first study considered the decreases in body mass and brain size that have been documented for a number of other primate lineages. Researchers tested putative ancestors for Homo floresiensis against these, using the high, medium and low estimates of its body mass. Results suggested that Homo erectus is only feasible as an ancestor for Homo floresiensis if the low estimate of 16 kg (35 lb.) is accepted. For the medium estimate of 24 kg (53 lb.), Homo habilis or the Dmanisi hominins are more feasible as ancestors. The high estimate is not compatible with any proposed scenario (Montgomery, et al., 2010).

The second study re-evaluated the brain size of Homo floresiensis using micro-CT scanning, and obtained an upwardly-revised estimate of 425 cc. This revised figure means that the degree of brain size reduction in relation to body mass is less, and therefore easier to explain. Nevertheless, if Homo erectus was the ancestor, the decrease in brain size is still too great to be explicable solely as a scaling downwards as body mass is downsized, and some other factor must have come into play. In an environment where food is scarce, and given that brain tissue is ‘expensive’ in metabolic terms, further reduction in brain size might have been advantageous. However, this could not be achieved without some loss of cognitive ability. The authors of the report suggested that cognitive abilities comparable to Homo habilis might have sufficed in an island habitat lacking dangerous predators (Kubo, et al., 2013).

While this ‘dumbing down’ scenario cannot be dismissed, it seems implausible. According to the widely-accepted ‘social brain hypothesis’ (Byrne & Whiten, 1988), the large primates of primates evolved in response to a need to predict the likely future social behaviour of their fellows, and base relationships upon these predictions. While Homo floresiensis would not have had to face dangerous predators, individuals would still need to interact with other group members. It is difficult to see that being less smart than one’s fellows could be anything over than a severe disadvantage, regardless of other circumstances.

I would therefore be inclined to the view that Homo erectus was not the ancestor of Homo floresiensis, and this view is supported by a number of studies considering the skeletal evidence. These have noted that while the cranial metrics were consistent with Homo erectus, the limb proportions of Homo floresiensis had more in common with Australopithecus garhi (Argue, et al., 2006) and the feet were a mosaic of primitive apelike and derived humanlike features. The big toe was fully in-line, albeit short, and the metatarsals followed a humanlike sequence in which the 1st (innermost) was the most robust (sturdily-built), followed by the 5th (outermost), then 4th, 3rd, and finally 2nd. The foot, though, was disproportionately long in comparison to that of a modern human; the lesser metatarsals (2nd to 5th) were long; and the outer toes were long and curved, unlike the short, straight toes of a modern human (Jungers, et al., 2009). The fact that the feet and limb proportions of Homo erectus were modern suggests that Homo floresiensis evolved from a species that was more primitive, such as Homo habilis.

References:

1. Brown, P. et al., A new small-bodied hominin from the Late Pleistocene of Flores, Indonesia. Nature431, 1055-1061 (2004).

2. Morwood, M. et al., Archaeology and age of a new hominin from Flores in eastern Indonesia. Nature431, 1087-1091 (2004).